(While I was typing, mouser also posted, but...) In addition to what mouser said:
Javaw is as 'hard' as
C#w, the mainstream .NET language, and does officially run on both Linux and Mac, next to Windows and a plethora of other OS's like Android (though with it's own programming model) and many embedded systems (think:
IoTw).
.NET languages can be (created, compiled and) run on Linux, and derivatives like Mac, using the
Mono compilers and runtime, but it is a kinda 'reverse-engineered' solution, as it originally wasn't supported by Microsoft, and still isn't fully feature-complete compared to the current .NET runtime, and some parts will even never be done for Mono.
The major IDE's for Java,
Eclipse and
NetBeans, are free. And there is a free community edition of
IntelliJ IDEA by Jetbrains next to a paid version, where the free edition is also the base for the new Android Studio from Google (that currently has Eclipse as it's foundation).
Next to that, solutions for
any programming dilemma in Java can easily be found using Google, while for .NET that can be a lot harder to find, or end up in a commercial solution.
(All of the above from personal experience)